Platelet-leukocyte interactions occupy a central role in the interface between thrombosis and inflammation[unreadable] and may contribute to intimal hyperplasia, atherosclerosis, and microvascular occlusion. We and others[unreadable] have demonstrated that the early recruitment of neutrophils by adherent platelets following arterial[unreadable] endothelial denudation injury correlates with the subsequent development of intimal hyperplasia. The[unreadable] recruitment of leukocytes by platelets may serve as a crucial switch in the injury response that converts[unreadable] the dominant biology to inflammation with leukocyte infiltration into the vessel wall and initiation of a local[unreadable] and systemic inflammatory response. We hypothesize that unique molecular determinants control the[unreadable] binding of neutrophils to platelets and their subsequent transmigration. The broad objective of this[unreadable] application is to define the molecular events that are triggered in neutrophils upon binding to platelets and[unreadable] then target those pathways in mice to delineate their relative contributions to the response to arterial[unreadable] injury and to neutrophil extravasation across inflammed endothelium. We have identified a signaling[unreadable] pathway in neutrophils that is initiated upon contact with P-selectin and appears to activate Rap1 and[unreadable] integrin alphaMbeta2. Src-family kinase (SFK) and Pyk2 serve as sensors of beta2 integrin engagement[unreadable] and stabilize firm integrin adhesion. Our data suggests that Pyk2 activity is regulated by[unreadable] phosphodiesterase-sensitive cAMP pools. In Specific Aim One, we will identify the molecular[unreadable] determinants of Pyk2-dependent signaling and the relative contribution(s) to physiologically relevant[unreadable] platelet-leukocyte interactions. In Specific Aim Two, we will establish a role for Rap1 in promoting[unreadable] platelet-leukocyte interactions and pathophysiologic vascular injury responses. In Specific Aim Three, we[unreadable] will identify unique controls for the platelet-leukocyte switch using animal models of acute inflammation[unreadable] and isolated cell systems. This work will define specific signaling events that are initiated in neutrophils[unreadable] upon adhesion to platelets and their relative contributions to acute inflammatory processes in potentially[unreadable] clinically-relevant models. Successful completion of the work should validate pharmacologic strategies[unreadable] aimed at blocking platelet-neutrophil interactions as a novel approach to treat or prevent atherothrombotic[unreadable] vascular disease.